1. Field of the Invention
The present invention relates to a method and apparatus for forming a face structure for a golf club head. More specifically, the present invention relates to a method and apparatus for forming a face structure composed of an interconnected reinforcement piece and a polymer material.
2. Description of the Related Art
Technical innovation in the material, construction and performance of golf clubs has resulted in a variety of new products. The advent of metals as a structural material has largely replaced natural wood for wood-type golf club heads, and is but one example of this technical innovation resulting in a major change in the golf industry. In conjunction with such major changes are smaller scale refinements to likewise achieve dramatic results in golf club performance. For example, the metals comprising the structural elements of a golf club head have distinct requirements according to location in the golf club head. A sole or bottom section of the golf club head should be capable of withstanding high frictional forces for contacting the ground. A crown or top section should be lightweight to maintain a low center of gravity. A front or face of the golf club head should exhibit high strength and durability to withstand repeated impact with a golf ball. While various metals and composites are known for use in the face, several problems arise from the use of traditional face structure and materials. In addition, material interaction of the golf club head and the golf ball during impact is an important factor for performance of the golf club.
The golf ball is typically composed of a core-shell arrangement with a thin polymer shell, or cover material such as ionomers, surrounding a rubber-like core. These polymeric materials exhibit compression and shear, stiffness and strength properties dependent upon strain (load), input frequency (time dependency of small linear strain), strain rate (time rate of loading including large nonlinear strains), and temperature. The compression and shear stiffness properties of polymeric materials are measured and classified in terms of a storage moduli (E′, G′) and a loss moduli (E″, G″), respectively. The storage moduli (E′, G′) represent the amount of compression and shear energy, respectively, stored during a complete loading cycle. For quasi-static loading, it is equivalent to the well known Young's modulus (E′=E) and shear modulus (G′=G=E/(2(1+ν)), where (ν) is the material Poisson ratio. For most polymers, the storage modulus increases significantly with strain, input frequency, and strain rate. For example, typical storage moduli for golf balls at low speed impacts, in the temperature range (50-100° F.), are E′ball=450-6,000 lb/in2 and G′ball=150-2,000 lb/in2. During high-speed impacts, in the temperature range (50-100° F.), the typical storage moduli are E′ball=9,000-50,000 lb/in and G′ball=3,000-16,500 lb/in2. The low speed impact represents a putting stroke or a soft pitch shot, while the high-speed impact represents a golf swing with an iron-type or a wood-type golf club head.
The loss moduli (E″, G″) represent the amount of compression and shear energy, respectively, dissipated during a cycle. For most polymers, the loss moduli also increase significantly with strain, input frequency, and strain rate, but the rate of increase can be very different than the aforementioned storage moduli. Finally, the magnitude of the loss moduli at a given strain, strain rate, frequency, or temperature typically vary from 0.005-2.0 times that of the storage moduli.
A loss (or damping) factor (ηE, ηG) or loss angle (δE, δG) for compression and shear are commonly defined as the ratio of the corresponding moduli;
            η      E        =                  Tan        ⁢                                  ⁢                  δ          E                    =                        E          ″                          E          ′                      ,          ⁢            η      G        =                  Tan        ⁢                                  ⁢                  δ          G                    =                                    G            ″                                G            ′                          .            These loss factors are an important measure of the damping capability (energy loss mechanisms) of the material. For most ball-type materials, (ηE≅=ηG) and magnitudes fall in the range of 0.005 (low energy loss) to 2.0 (high-energy losses), where magnitudes clearly depend upon polymer composition, strain, input frequency, strain rate, and temperature. As a comparison, the loss factors (energy loss mechanisms) in a metallic face of a golf club head are on the order of 10-100 times smaller than that of a golf ball. For most elastomeric polymer materials operating below the glass transition region, the Poisson ratio is fairly constant with (ν=0.4-0.5), while for stiff polymers acting at or above the glass transition region (ν=0.3-0.33).
Thus, during impact of the golf ball with the golf club head, a significant portion of impact energy is lost as a result of the large deformations (0.05 to 0.50 inches) and deformation rates of the high damped golf ball materials, as opposed to the small deformations of the low damped metallic club face (0.025 to 0.050 inches) materials. A larger portion of this impact energy is lost in the golf ball because the magnitude of the deformation, the deformation rate, and energy loss mechanisms are greater for the golf ball than the face of the golf club head.
Application of hard polymers to the face of the golf club head represents a traditional structure of natural wood golf club heads, where a hard insert material centrally located in the face of the golf club and requiring an exacting fit between two or more distinct elements. The hard insert must be manufactured to a close tolerance to fit within a recess in the face of the golf club, and high surface hardness is less efficient in transferring energy to the golf ball during impact with the golf club. A homogeneous face structure is simpler to manufacture but is limited to the inherent material properties of the single material comprising the face structure. The present invention achieves a more efficient energy transfer during impact while maintaining a simple construction.
The use of a polymer coating or layer on the face of a metal driver has been shown to increase durability and the coefficient of restitution (“COR”) of the driver through the compliance of the polymer layer. However, when a golf ball impacts the face at angles that are not normal to face, such as the case with lofted drivers, shear deformation of the polymer coating introduces energy losses that reduce or eliminate the COR benefit provided by the polymer coating.
The United States Golf Association (“USGA”) has set forth a test for determining the flexibility of a golf club head. The test procedure is available at www.USGA.org under Procedure For Measuring The Flexibility Of A Golf Clubhead. The test uses a pendulum testing apparatus to determine the characteristic time (“CT”) between a clubhead and a pendulum during impact. In order to be in conformity with the Rule of Golf as set forth and interpreted by the USGA, the CT must not be greater than 239 milliseconds plus a tolerance of 18 milliseconds for a total of 257 milliseconds. U.S. Pat. No. 6,505,498 and U.S. Patent Publication Number 2004/0182131 each disclose a pendulum testing apparatus which may be used to determine the CT of a clubhead.
The CT test replaces a coefficient of restitution (“COR”) test that was previously used by the USGA to render decisions on the conformity or non-conformity of golf club drivers. The COR test had a limit of 0.83.